This invention relates to a method and apparatus for developing presensitized offset printing plates. More particularly, this invention relates to a method and apparatus for developing presensitized offset printing plates of the type that are individually processed with a substantially virgin aqueous solution containing an alkali Silicate, which aqueous solution can after use be disposed of without causing the problem of sludge formation in the developing solution or subsequent scumming of the printing plates.
Presensitized offset printing plates can be developed by various methods among which is one that comprises diluting a concentrated solution of an alkali silicate and supplying the diluted solution onto the surface of printing plates. Ordinary tap water is used to dilute the concentrated solution of an alkali silicate. However, if tap water or well water is directly used as the diluent, calcium ions contained in that water will bind with the silicic acid in the developing solution to form a precipitate, which can potentially plug the pump for feeding the diluted solution or the temperature controlling heater. The precipitate is also deposited on the presensitized plate being processed, which will eventually foul the print.
Unexamined Published Japanese Patent Application No. 54341/1983 discloses a technique in which a specified amount of a water softener capable of at least 50% sequestration at the pH of a developing solution of interest is contained in that developing solution which contains an alkali agent, etc., whereby the generation of a slimy product can be prevented that would otherwise occur if the organic high polymer (having an acid value of 10-200) contained in the light-sensitive layer dissolved into the developing solution and underwent an ionic crosslinking reaction with calcium and/or magnesium ions. Unexamined Published Japanese Patent Application No. 190952/1983 discloses a technique in which a chelating agent is contained in a developing solution (i.e., an aqueous solution of an alkali silicate), whereby the generation of insoluble calcium silicate or magnesium silicate is prevented that would otherwise occur if the alkali silicate in the developing solution reacted with calcium or magnesium ions in the diluting water.
However, these conventional techniques are still unsatisfactory in their effectiveness for preventing the formation of a precipitate caused by the binding of silicic acid with calcium ions in the diluting water particularly in the case where they are applied to a development system of the type that processes presensitized offset printing plates individually with a substantially virgin developing solution, which is disposed of after use. This probably due to the following two reasons. First, compared to the case of recycling the developing solution, more of the working solution which is a mixture of the concentrated developing solution and diluent water will pass through the feed pump or the temperature controlling heater. Second in the use of a recycled developing solution, the contents of calcium and magnesium in the developing solution will decrease in proportion to the amounts in which are used to form sludge but this does not occur in the method under consideration (i.e., a substantially virgin developing solution is supplied for processing individual printing plates and is disposed of after use), so that the developing solution passing through the feed pump or the temperature controlling heater will contain invariably large amounts of calcium and magnesium, which eventually leads to the formation of more sludge.